The present invention is directed to an automatic slack adjusting mechanism for a vehicle brake system. More specifically, the invention is directed to a slidable brake actuator plunger assembly responsive to movement beyond a predetermined distance to automatically adjust for slack in the system upon brake releasing movement of the plunger assembly independently of system reaction force released through the plunger assembly.
The invention is particularly useful in combination with an automatic brake adjusting mechanism of the type disclosed in U.S. Pat. No. 3,068,964 issued to W. J. Williams et al on Dec. 18, 1962. That mechanism is comprised of an adjusting rod threadably received internally of an adjustng nut having a helical toothed external surface portion. The adjusting nut and rod are carried internally of a sleeve slidably adjusting nut and rod are carried internally of a sleeve slidably mounted in a spider housing. One end of the rod is non-rotatably engaged with the end of a brake shoe. A wedge actuator of the type disclosed in U.S. Pat. No. 3,037,548 issued to F. T. Cox et al on June 5, 1962 engages the other end of the plunger assembly to move the plunger assembly axially in the spider housing and the brake shoes and lining into frictional contact with a brake drum. Detent means comprising a spring biased pawl having buttress teeth threadedly engaging the helical toothed surface portion of the adjusting nut are also carried by the spider housing.
In that mechanism, when the running clearance increases due to lining wear, the additional axial movement required for the lining to contact the drum causes the pawl to skip a helical tooth on the adjusting nut. Then, when the brake is released, the spring biased return movement of the brake shoes and a reaction force produced by energy stored in the system are transmitted through the rod and the adjusting nut causing the helical teeth on the adjusting nut and the nut to rotate about the buttress teeth provided to the pawl. Rotary movement of the nut relative to the rod moves the rod axially outward of the assembly thereby increasing the overall length of the plunger assembly and returning the brake shoes and lining to the position which provides the desired running clearance between the lining and the drum.
Although the foregoing adjusting mechanism has proven successful in use with wedge actuated drum type brake systems, the automatic adjusting mechanism is subjected to the system reaction force upon release of the brake and the minimum axial length of automatic adjustment is limited by the size and the helix angle of the teeth which must be provided to the pawl and adjusting nut to withstand that force. These problems have become more significant with the advent of disc type brake systems which employ a U-shaped caliper supporting the pad or lining carrying brake shoes on opposite sides of a rotor. In such systems the legs of the caliper expand outwardly as the brake is applied. This distortion produces a substantial reaction force which is transmitted back through the actuating and adjusting mechanism when the brakes are released and may shear the teeth engaged between the pawl and adjusting nut of the previously described automatic adjusting mechanism.
The pressure required between the pads and rotor in a disc brake system is also greater than that required between the brake shoe linings and drum in the conventional drum system. In a wedge actuated disc brake system the additional pressure can be developed by decreasing the wedge angle of the wedge actuator. This increases the mechanical advantage between the wedge actuator and the plunger assembly, but is also reduces the wedge induced axial movement of the plunger assembly and requires a finer increment of automatic adjustment to maintain the desired running clearance between the brake shoe pads and the rotor.
The plunger assembly of the present invention avoids the foregoing problems by providing means independent of the system reaction force for biasing an adjusting nut to the brake released position and axially movable rotary driving means between the adjusting nut and the rod to permit the system reaction force to move the rod relative to the adjusting nut as the brake is released.